1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to the manufacture of tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in the manufacture of color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask is supported in high tension a predetermined distance from the inner surface of the faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or "parallax barrier," which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension, typically 30 lb/inch. The support means must be of high strength so the mask is held immovable; an inward movement of the mask of as little as 0.0002 inch can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal expansion compatible with the glass, and by its composition, be bondable to glass. Also, the support means should be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the surface is composed should be adaptable to machining or other forms of shaping so that it can be contoured into near-perfect flatness so that no voids between the metal of the mask and the support structure can exist to prevent the positive, all-over contact required for proper mask securement.
To forestall cracking or spalling of the glass of the faceplate resulting from the stress inherent in the cementing of the support structure to the glass of the faceplate, it is essential that the coefficients of thermal contraction ("CTC") of the glass of the faceplate, the metal of the tension mask support structure, and the devitrifying solder glass (known coloquially as "frit"), used as the cement, be compatible. For example, the metal used in conjunction with the support structure may comprise Alloy No. 27 manufactured by Carpenter Technology of Reading, Pa.; this material has a CTC of approximately 105 to 109.times.10.sup.-7 in./in./degree C. over the range of the temperatures required for devitrification--from ambient temperature to 450 degrees C. Faceplate glass such as that supplied by Corning Glass Works, Corning, N.Y. under the designation 9068, has a CTC of approximately 100.times.10.sup.-7 in./in/degree C. from 300 degrees C. to room temperature. Solder glass 7590, also supplied by Corning Glass Works, has a CTC of 97.times.10.sup.-7 in./in./degree C. from 300 degrees C. to room temperature. This range of CTC's is about as compatible as it is possible to achieve with this range of materials, and makes feasible the cementing of them together under the wide temperature variations required in tube manufacture.
It is desirable in some applications to provide a mask support system, including cement, having a composition effective to place the glass beneath the support system into a predetermined degree of tension. This concept is described and claimed in referent copending application Ser. No. (D5937), of common ownership herewith.
It has been found that the cementing of a support structure to the glass of the faceplate can appreciably distort the faceplate if the mismatch is great enough. In the practical utilization of interchangeable mask systems, such as those set forth in referent copending application Ser. No. 223,475 and its two continuations-in-part: Ser. Nos. 370,204 and 405,378, it is important that the faceplate be as flat as possible, and that it remain flat throughout the wide temperature excursions incident to the process of cathode ray tube manufacture. Any excessive out-of-plane distortion may bring into question the feasibility of interchangeable mask systems in flat tension mask tubes.
Significant factors in the manufacture of a tension mask support structure include: (1) the cost of the materials of the structure; (2) the compatibility of the composition of the support structure with the glass of the faceplate; and (3) the flatness/parallelism of the structure.
The requirement for flatness and parallelism remains the same regardless of the length of the rails, which in turn is determined by the size of the tube; the longer the rail, the greater the problems in its fabrication, and the greater its cost. Also, the longer the rail, the greater any effect of the incompatibility of the materials of rail and faceplate, and hence the greater the resulting distortion of the faceplate.
In consequence, there has been a very real limitation on the potential size of flat tension mask cathode ray tubes. It is an objective of the present invention to resolve the size-limitation and the related problems.
2. Prior Art
U.S. Pat. No. 4,730,143 to Fendley, of common ownership herewith, and its reissue application Ser. No. (D5261-R), disclose a color cathode ray tube having a faceplate-mounted mask support structure with a welded-on, high-tension foil shadow mask. The faceplate of the tube has on its inner surface a centrally disposed phosphor target surrounded by a peripheral sealing area adapted to mate with a funnel. A separate metal faceplate frame is secured to the inners surface of the faceplate between the sealing area and the target. The separate metal frame supports a welded-on tension foil shadow mask a predetermined distance from the inner surface of the faceplate. The separate face-mounted frame has, according to the '143 invention, a plurality of slurry-passing structures contiguous to the inner surface of the faceplate for passing any surplusage of slurry during the radial-flow, slurry-deposition process used in screening the faceplate. In one embodiment of the invention, a faceplate-mounted metal frame is shown as being discontinuous ("broken") or segmented. Gaps in the metal frame provide for the passage of slurry used in screening, and the discontinuities in the metal of the support structure are said to compensate for differences in the coefficients of thermal expansion or contraction of the metal of the support structure and the glass of the faceplate. The foil shadow mask is indicated as being welded directly to the metal of each discrete part of the support structure.